Typically, tire tread rubber formulations include a blend of rubbers of varied glass transition temperatures. Rubbers having low glass transition temperatures normally improve treadwear and rolling resistance and rubbers having high glass transition temperatures typically improve traction characteristics. However, it is normally difficult to improve the rolling resistance of a polymer system that contains a large amount of a polymer having a high glass transition temperature (Tg), such as an isoprene-butadiene rubber having a glass transition temperature which is within the range of about -50.degree. C. to about 0.degree. C. The high hysteresis characteristics of the rubber having the high glass transition temperature reduces rebound. The level of filler utilized can be reduced to improve rebound characteristics, but this increases cost and affects other compound properties. Increasing the cure level improves the rebound values, but it also affects other compound properties.
Low rolling resistance is an important characteristic of tires because good fuel economy is virtually always an important consideration. To attain good rolling resistance, rubbers in the tire tread compound having high glass transition temperatures also generally have high molecular weights (a Mooney ML 1+4 viscosity of about 70 to about 90). Isoprene-butadiene rubber having a low glass transition temperature can be utilized in such tire tread compounds to improve processing. However, when these low Tg isoprene-butadiene rubbers have a high molecular weight, the rubber compound becomes very difficult to process. On the other hand, decreasing the molecular weight of the isoprene-butadiene rubber improves processability but increases rolling resistance. The inclusion of low molecular weight isoprene-butadiene rubber in the tire tread rubber compound also increases cold flow which results in processing difficulties at tire manufacturing plants.
Good treadwear is also an important consideration because it is generally the most important factor which determines the life of the tire. The traction, treadwear and rolling resistance of a tire is dependent to a large extent on the dynamic viscoelastic properties of the elastomers utilized in making the tire tread. In order to reduce the rolling resistance of a tire, rubbers having a high rebound have traditionally been utilized in making the tire's tread. On the other hand, in order to increase the wet skid resistance of a tire, rubbers which undergo a large energy loss have generally been utilized in the tire's tread. In order to balance these two viscoelastically inconsistent properties, mixtures of various types of synthetic and natural rubber are normally utilized in tire treads. For instance, various mixtures of styrene-butadiene rubber and polybutadiene rubber are commonly used as a rubber material for automobile tire treads. However, such blends are not totally satisfactory for all purposes.
Rubbers having intermediate glass transition temperatures (-70.degree. C. to -40.degree. C.) compromise rolling resistance and treadwear without significantly increasing traction characteristics. For this reason, blends of rubbers having low glass transition temperatures and rubbers having high glass transition temperatures are frequently utilized to attain improved traction characteristics without significantly compromising rolling resistance or treadwear. However, such blends of rubbers having low glass transition temperatures and rubbers having high glass transition temperatures exhibit poor processability. This major disadvantage associated with such blends has greatly hampered their utilization in making tire tread compounds.
Tin-coupled polymers are known to provide desirable properties, such as improved treadwear and reduced rolling resistance, when used in tire tread rubbers. Such tin-coupled rubbery polymers are typically made by coupling the rubbery polymer with a tin coupling agent at or near the end of the polymerization used in synthesizing the rubbery polymer. In the coupling process, live polymer chain ends react with the tin coupling agent thereby coupling the polymer. For instance, up to four live chain ends can react with tin tetrahalides, such as tin tetrachloride, thereby coupling the polymer chains together. However, rubbery polymers having glass transition temperatures of greater than about -50.degree. C. are difficult to couple with tin compounds, such as tin tetrahalides. It is accordingly not commercially feasible to couple rubbery polymers having a glass transition temperature which is within the range of about -50.degree. C. to about 0.degree. C.